Field of the Invention
The present invention relates to a sheet processing apparatus that processes a sheet on which an image is formed and an image forming apparatus including the sheet processing apparatus.
Description of the Related Art
Recently, a sheet processing apparatus has been proposed in which a sheet or a sheet bundle acquired by piling a plurality of sheets on which images are formed is once discharged to a processing tray so as to be aligned and is stapled and then, is discharged to a stack tray by a discharge belt to which projections are attached (U.S. Pat. No. 5,762,328).
In the sheet processing apparatus disclosed in U.S. Pat. No. 5,762,328, after a sheet or a bundle of sheets that is discharged to the processing tray and is aligned in the width direction intersecting the discharge direction of a sheet by one pair of aligning plates is discharged to the stack tray by the discharge belt to which projections are attached, the next sheet is discharged to the processing tray.
In addition, in the sheet processing apparatus disclosed in U.S. Pat. No. 5,762,328, when the sheet bundle is moved by the discharge belt, the sheet bundle is moved by the discharge belt while both end portions of the sheet bundle in the width direction are pressed by one pair of aligning plates so as not to allow the sheets to deviate from each other during the movement.
In addition, in the case of a job in which a plurality of sheet bundles is consecutively discharged to a stack tray, so-called offset discharge is known in which a sheet bundle on a processing tray is moved from the center in the width direction to one side or the other side by a predetermined amount by aligning plates, and then, the sheet bundle is discharged to the stack tray.
However, in order to perform the offset discharge by connecting the above-described sheet processing apparatus to an image forming apparatus having high productivity, any one of the aligning plates is in a discharge range of a sheet to be discharged to the processing tray. Accordingly, unless the discharge of a preceding sheet bundle has been completed, the aligning plate cannot be moved to a retraction position that is used for receiving the next sheet. Therefore, in order to save a retraction time for moving the aligning plate to the retraction position, the productivity has to be lowered, and accordingly, an operation of discharging a sheet to the processing tray, and an operation of aligning a sheet on the processing tray are inefficient.